Currently, whole liver transplantation is the primary method used to treat end stage chronic liver failure. With the marked shortage of available donor livers, there has been considerable interest in identifying stem or progenitor cells can restore liver mass and function. We have demonstrated that fetal liver epithelial cells can repopulate up to 30% of parenchymal mass in normal liver, produce new hepatocytes and bile ducts and continue to proliferate for up to one year after cell transplantation. These are all properties of stem cells and we refer to these cells as fetal liver stem/progenitor cells (FLSPC). We hypothesize that use of a normal liver based cell transplantation system, such as the one we have established, is critical in determining the stem cell potential of isolated cells and cell lines and in identifying essential factors that contribute to their proliferation and differentiation in the liver. Recently, we discovered that fetal liver cells replace host hepatocytes by cell competition, a process originally described in Drosophila. Within this context, experiments are proposed: 1. To study the hypothesis that engraftment of fetal liver stem/progenitor cells into the hepatic parenchyma represents a rate limiting step in liver repopulation. These studies will utilize mouse and rat fetal liver stem/progenitor cells that have been purified to 95% homogeneity using immunomagnetic beads (MACS). 2. To study the hypothesis that modulating cell competition and understanding the circumstances under which this occurs will lead to identifying optimal conditions for liver repopulation by transplanted FLSPC. How long does repopulation/competition continue? Does the age of the host recipient at the time of cell transplantation influence the extent of repopulation? Is competition always driven by increased apoptosis in the host liver? and Can we block liver repopulation by knocking down expression of the anti-apoptotic gene, survivin, in FLSPC prior to their transplantation? 3. To identify specific genes, signaling pathways and mechanisms that regulate cell competition during mammalian liver repopulation. Our hypothesis is that the competitive advantage of transplanted FLSPC over host hepatocytes derives from overexpression of anti-apoptotic genes in these cells, coupled with augmented apoptosis of host hepatocytes through the extrinsic cell death pathway. We will identify the specific genes and signaling pathways involved by isolating RNA from actively proliferating transplanted cell clusters and surrounding host liver by laser-capture micro dissection and studying specific gene expression by RT-PCR, qRT-PCR and high density gene expression microarrays and 4) To directly demonstrate critical stem cell properties of fetal liver epithelial progenitor cells in vivo, including their ability to grow clonally, their bipotency and their capacity for self-renewal. The overall goal of these studies is to find methods to enhance liver repopulation by transplanted hepatic derived cells that will ultimately lead to clinical application in humans. Public Health Relevance: This project is directed toward optimizing repopulation of the adult rat liver with fetal liver stem/progenitor cells and in determining the critical steps, mechanism and genes involved in cell competition, which regulates this process.